In modern furnaces of the forced air heating type (oil, gas, electric, hot water, and steam), it is usual for the controls to initiate the burner or heater function and thereafter, upon achieving a thermal set condition, for example in the plenum or heat ducts, to then initiate the blower function. The heat cycle continues until the house thermostats reach the upper set point and the controls stop the hot air blower and the heat source or furnace. Actually, heat is increasing in the plenum over ambient air conditions substantially at the instant of application of the heat source as, for example, from combustion in the furnace and the heat is residually retained in the plenum or heat exchangers after the furnace is turned off and until the heat surfaces reach ambient temperatures.
Regular controls do not treat the situation at start-up by starting the warm air blower and do not treat the situation, as in the present invention, to strip the residual heat after furnace shutdown. Without the present invention, substantial loss of available heat occurs directly up the chimney and as a consequence causes sucking of combustion air into the house or area heated upon commencement of combustion and before commencement of the plenum blower.
Investigations conducted to determine the amount of heat saving have been extraordinarily encouraging and when the contribution of the present invention is compared against regular controls, the forced air heating systems in which the present invention has been utilized have demonstrated substantial savings of heat and consequent economy to the users. As applied to gas furnaces, the electric consumption held level and a gas savings of about 27,000 cubic feet resulted in a heating season. Maximum savings is realized when the increase in unit costs of gas and oil are contemplated. This is also true because the gradual heat rise at the start of the cycle and the relatively high thermal residuals at shutdown are utilized. The system is applicable to gas fired, oil fired, and electric warm air furnaces as well and on the basis of these findings it is possible to anticipate a fuel savings of between about seven to thirty percent over the experiences of regular controls of varying sophistication. These accomplishments require no physical alteration of existing forced air heating plants and with the present invention this is achieved by simple addition to existing controls.